在codeblocks里三段代码输出，请分析结果并说明原因

#include<cstdio>
#include<cmath>
#include<cstring>
int main()
{
    int a = 3;
    int b = pow(10,a);
    printf("%d",b);
    return 0;
}

#include<cstdio>
#include<cmath>
#include<cstring>
int main()
{
    int a = 3;
    int b = pow(10,a-1);
    printf("%d",b);
    return 0;
}

#include<cstdio>
#include<cmath>
#include<cstring>
int main()
{
    int a = 3;
    int b = pow(10,a-2);
    printf("%d",b);
    return 0;
}